(1) Field of the Invention
The present invention relates to a process for the preparation of nitrites. More particularly, the invention relates to a process for preparing an alkali metal nitrite or ammonium nitrite by electrolytic reduction of an aqueous solution containing a nitrate of an alkali metal, such as sodium nitrate, potassium nitrate or ammonium nitrate.
(2) Description of the Prior Art
Conventional processes for preparing nitrites will now be described by reference to the production of sodium nitrite as a typical instance of the nitrite. According to one known process, sodium nitrate is reduced by lead, whereby sodium nitrite is formed by the reaction represented by the formula: EQU NaNO.sub.3 +Pb=NaNO.sub.2 +PbO.
According to another known process, gaseous nitrogen oxide is absorbed in a solution of sodium hydroxide or sodium carbonate, whereby sodium nitrite is formed by the reaction represented by the formula: EQU Na.sub.2 CO.sub.3 +2NO+1/2O.sub.2 =2NaNO.sub.2 +CO.sub.2.
In the process using lead as the reducing agent, in order to remove lead incorporated in sodium nitrite, crystallization should be repeated several times. The process using gaseous nitrogen oxide is defective in that, if the absorbing liquid is acidic, nitrous acid and nitric acid are formed as by-products.
The inventors conducted research with a view to developing a process in which nitrites can advantageously be prepared industrially and found that, when an aqueous solution containing a nitrate is subjected to electrolytic reduction under specific conditions, the corresponding nitrite can advantageously be prepared industrially.
It is known that electrolytic reduction of nitrate ions is difficult to effect under ordinary electrolysis conditions. For example, in "LES REACTIONS ELECTROCHIMIQUES METHODS ELECTROCHIMIQUES D'ANALYSE", G. Charlot states that on a mercury electrode, direct reduction of nitrate and nitrite ions is very slow and is caused only at an electric potential which is approximately that at which supporting electrolytes are reduced, that is, in a solution containing Na.sup.+ or K.sup.+ ions, nitrate and nitrite ions are not electrically active. In short, G. Charlot feels that electrolytic reduction of nitrates is difficult.
Electrolytic reduction of an aqueous solution containing ammonium nitrate has been attempted on a laboratory scale. For example, in Research Group Report AERE-R4393(1963), of the U.K. Atomic Energy Authority, it is taught that ammonium nitrate can be decomposed to nitrogen and water according to reactions represented by the formulae (1) and (2): EQU NH.sub.4 NO.sub.3 +2e+2H.sup.+ .fwdarw.NH.sub.4 NO.sub.2 +H.sub.2 O (1) EQU NH.sub.4 NO.sub.2 .fwdarw.N.sub.2 +2H.sub.2 O (2)
In this reference, results of experiments conducted for finding conditions enabling decomposition of ammonium nitrate to nitrogen and water at temperatures approximating the boiling point are disclosed. In this reference, it is concluded that in the electrolytic reduction of the formula (1), the pH of the catholyte is gradually elevated with the advance of the electrolysis and the current efficiency is reduced substantially to zero if the catholyte is neutral or alkaline, and therefore, it is necessary to maintain the pH of the catholyte not higher than 1 during the electrolysis.
Japanese Patent Laid-Open Application No. 56375/75 discloses a process for the electrolytic reduction of an aqueous solution containing ammonium nitrate. This process is characterized by no use of a permeable membrane, and as clearly described in the specification of this application (page 4, last line to page 5, line 4), the production of nitrites is not intended and conditions for preparing nitrites are not disclosed at all.